You can always buy one or two 64 GB SD cards if you run out of memory. Not that you should _plan_ to do that, but if you bought 128GB, and a few months or years later you run out of room, that's better than nothing.
Should be pointed out that the speed of those cards are much slower than the onboard SSD... even an external USB hard drive is faster. The fastest 128GB SDXCs also run around $150-ish, so its quite a bad trade-off really.
Might as well pony up the extra bucks for the 256GB SSD if anyone expects 128GB won't be enough.
Single OS, no Bootcamp/VMs and not a ton of data stored (some people have huge music/video libraries) 128GB shouldn't be an issue. If you have a pretty complete Windows install on top of OSX, then you can be pushing the boundaries of 128GB and yes it might fit without data being stored much onboard, but the space juggling just isn't worthwhile/productive.